The reserves in this sector are huge and over extremely large areas formerly not recoverable such as coal streaks, fine sand or schist. In the USA, these unconventional resources increased from 28% in 1998 to 46% of the total natural gas produced in the USA. The main huge basins are known as Barnett Shale, Fayette Ville Shale, Mowry Shale, Marcellus Shale, etc.
The production techniques have often evolved from vertical wells to horizontal wells, reducing the number of production wells. However, in all cases, the permeability is insufficient to naturally produce a quantity of gas making the operations cost-effective.
The permeability and production areas therefore need to be increased by way of stimulation operations and fracturing in particular.
Hydraulic Fracturing
Hydraulic fracturing is designed to create an additional level of permeability and an increased gas production surface areas. Indeed, low permeability, natural barriers of compact layers, impermeability due to drilling operations greatly limit production.
These fracturing operations started in 1960 in the Appalachian Mountains and today over one million operations have taken place in the USA.
Engineering, design and implementation technologies have become increasingly sophisticated and require equipment enabling to perform these operations in shorter times with detailed analysis of the results.
Specific simulators are capable of forecasting very accurate results.
Fracturing Operation
These operations involve injecting high pressure water at a very high flow rate to create fractures within the production wells.
When the fractures are open, sand or beads of a particular size are injected to prevent these fractures from closing and to maintain the production surface areas thus created.
The injection water usually contains a significant amount of additives, each with a defined function. Generally speaking, the injected composition includes:                clay swelling inhibitors such as potassium chloride        biocides to prevent bacteria, particularly sulphate-reducing bacteria, from developing and capable of forming viscous masses reducing the injection areas. Aldehydes are the most commonly used biocides as glutaraldehyde, acrolein, etc. . . .        oxygen scavengers such as ammonium bisulphite to prevent the destruction of other components due to oxidation and corrosion of the injection lines,        corrosion inhibitors to protect the lines from oxidation by residual quantities of oxygen. Preference is given to N, n dimethylformamide,        lubricants such as petroleum distillates,        gelling agents such as guar gum and hydroxyethyl cellulose,        iron-chelating agents such as citric acid, EDTA, phosphonates,        scale control products such as phosphates, phosphonates or ethylene glycol,        friction reducers to reduce the pressure drop in turbulent media and significantly increase the flow rate for the same power and pipe diameter.        
Acid treatment is often performed before or during injection to treat cementation residue or to dissolve part of the rock in place in order to provide better access to the field. The sand injected is of a particular size, usually between 40 and 100 mesh.
Friction Reducer
The drag reduction (friction reduction) phenomenon in turbulent flow was discovered by B. A Toms in 1946 (“Toms effect”) even though previous descriptions ascertained that certain additives seemed to enhance high speed flows.
These products are therefore used to reduce pressure drop in line.
When the medium is made up of hydrocarbons, oil-soluble polymers are used, particularly polymethyl methacrylate or carbohydrate polymers.
When the medium is water, very high molecular weight polymers such as polyacrylamides and ethylene oxide polymers are used. Preference is generally given to polyacrylamides for cost and sourcing reasons.
Polyacrylamides
Initially developed for flocculation applications (water treatment, mineral treatment, paper mills, etc), polyacrylamides are increasingly used in other applications, and especially where their thickening effect on water based system is desired.
This development is of particular interest for Enhanced Oil Recovery, Cosmetics and Thickeners for textile printing, etc.
In the description hereinafter and the subsequent claims, the generic term of polyacrylamides covers acrylamide homopolyers as well as all the copolymers containing both acrylamide and other monomers such as:                acids such as (metha)acrylic acid, AMPS (2-Acrylamido-2-methylpropane sulfonic acid), etc        cationic monomers such as the ammonium salts of trimethylethyl methacrylate, diallyl dimethyl ammonium chloride, etc        
These polymers can be modified by:                branching with branching agents in sufficiently low quantities to maintain the solubility        Hydrophobic monomers giving associative polymers        Monomers with both anionic and cationic charges (zwitteronic monomers)        Associations of anionic and cationic monomers (amphoteric), etc        Monomers giving comb polymers.        
The Person skilled in the art is well aware of these sets of products described in numerous patents.
For friction reduction, the polymers used must have the highest molecular weight possible while keeping very good solubility.
The polymer of choice is an acrylamide sodium acrylate copolymer (70% 30% molar ratio) with a molecular weight close to 20 million.
For their ease of handling, reverse emulsions (W/O) of these polymers are used with a concentration of active matter between 20 and 50%. These emulsions are stable in storage for about 6 months and dissolve very quickly (few minutes) in water under stirring.
They allow an in line injection in the fracturing water before the high pressure pump and are practically fully dissolved in a few minutes both immediately and in the injection line.
The quantities of polymers injected can be between 250 and 2,000 ppm of active matter and the fracturing time between 2 and 15 hours with several stages of injections (2 to 20).
The use of emulsions is today generalised over the entire territory of USA-Canada-Mexico and is spreading into other countries according to new reserves to be exploited.
Powder Polyacrylamides
Environmental pressure has become increasingly strong both in terms of the water used for this operation and the water produced.
The constant demand is to only consume the water from the field and to treat the water produced which is made up of 60 to 80% of the water injected.
The use of powder polymers avoid injecting both hydrocarbons and surface active agents to be treated in the water produced.